The present invention relates to a liquid crystal device used as a display for personal computers, etc., and a driving method therefor.
In recent years, liquid crystal devices have been frequently used, and most of them are TN (twisted nematic)-mode liquid crystal devices using a liquid crystal cell comprising a pair of substrates provided with alignment films having rubbing directions which are deviated from each other by 90 deg. and a nematic liquid crystal disposed between the substrates. There is also known a liquid crystal device wherein a nematic liquid crystal is placed in a splay alignment state between a pair of substrates provided with alignment films having identical rubbing directions. Further, a type of cell (xcfx80-cell) wherein the above-mentioned liquid crystal placed in a splay alignment is re-aligned into a bend alignment by applying a voltage thereto so as to provide an improved response speed, was disclosed by P. J. Bos, et al in 1983. Further, a system (OCB cell) wherein such a bend alignment cell is combined with phase compensation to provide an improved viewing angle characteristic was disclosed by Uchida, et al., in 1992.
In such a bend alignment-type nematic liquid crystal device aims at suppressing a back-flow phenomenon in response of liquid crystal to provide improved and high-speed responsiveness, but has involved several problems for its commercialization.
One of such problems is that an electric field treatment is required for transforming the splay alignment into the bend alignment. The splay-bend alignment transformation is not continuous, and a disclination line is present between the two alignment states, so that a process including nucleation and its growth is necessary. In such a process, it is difficult to cause nucleation in all the regions and the control of the nucleation threshold is difficult, so that a high voltage application is required for the electric field treatment. Further, the velocity of growth of the bend alignment regions formed by the nucleation is larger at a higher voltage application, and a time of several seconds to several minutes is required at a low voltage. Another problem is that, in an actual matrix-structured cell, the bend region does not readily grow between pixel electrodes. Several studies have been made regarding manners of voltage application in active matrix-type cells using TFTs (thin film transistors) (e.g., IBM, IDW 1996, p. 133, Initialization of Optically Compensated Bend-mode LCDsxe2x80x9d, and Japanese Laid-Open Patent Application (JP-A) 9-185032). A further problem is that the bend alignment is restored to splay alignment if the bending voltage is removed, so that a bending treatment is required anew for re-use.
In view of the above-mentioned problems, an object of the present invention is to provide a liquid crystal device utilizing a bend alignment, which is improved in (1) an electric field treatment for transforming splay alignment into bend alignment, and (2) a re-bending treatment before use.
Another object of the present invention is to provide a liquid crystal device requiring a lower re-bending voltage and a lower holding voltage for holding or retaining the bend alignment state.
A further object of the present invention is to provide a driving method for such a liquid crystal device.
According to the present invention, there is provided a liquid crystal device, comprising: a pair of substrates each having thereon an electrode and an alignment film, and a layer of nematic liquid crystal disposed between the substrates, wherein
at least one alignment film includes a homeotropic alignment film region capable of aligning longer axes of liquid crystal molecules in contact therewith to form an angle of 80-90 deg. with respect to the associated substrate surface, and another alignment film region capable of placing the liquid crystal in contact therewith in an alignment state capable of conversion into a bend alignment on application of a bending voltage.
The present invention also provides a driving method for the above liquid crystal device, comprising switching between a twist alignment state and a bend alignment state of the liquid crystal to effect a display based on a difference in optical characteristic between the twist and bend alignment states.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.